38
A. Shah
3.2 Conversion of Light into Electrical Carriers
by a Semi-conductor Diode
3.2.1 Absorption and Energy Conversion of a Photon
When light illuminates a solar cell, the semiconductor material absorbs photons;
thereby, pairs of free electrons and holes are created (see Fig. 3.1). However, in
order to be absorbed, the photon must have an energy E ph = hν (where h is Planck’s
constant and ν the frequency of light) higher or at least equal to the bandgap energy E g
of the semiconductor. The bandgap energy is the difference in energy levels between
the lowest energy level of the conduction band (E C ) and the highest energy level of
the valence band (E V ) (Fig. 3.2). For a given semiconductor E g is a constant, which
only slightly depends on temperature. Table 3.1 gives the values of E g for various
amorphous and crystalline semiconductors at T = 25 °C.
Depending on the energy of the photon and on the bandgap energy of the solar
cell material, three cases can occur:
1. E ph = E g : in this case, the photon can be absorbed and will then generate a single
electron-hole pair (Fig. 3.3), without loss of energy.
2. E ph > E g : in this case, the photon can be easily absorbed and will then create a
single electron-hole pair. The exceeding energy E ph − E g is rapidly transformed
into heat (thermalisation; Fig. 3.4).
3. E ph < E g : the energy of the photon is not high enough to be absorbed. The photon
will be either reflected or absorbed elsewhere, and its energy is lost.
free hole
free electron
Photon
Semiconductor
Fig. 3.1 Creation of an electron-hole pair through absorption of a photon of energy E ph = hν.
Reproduced from [4] with the kind permission of the EPFL Press
Conduction band
E C
Valence band
E V
E = E -E
C V (gap)
g
Fig. 3.2 Bandgap energy E g of a semiconductor. Reproduced from [4] with the kind permission
of the EPFL Press